Method and apparatus for prevention of slug formation in the casting of film support and sheeting



Dec. 3, 1963 A. E. CZERKA 3,112,528

METHOD AND APPARATUS FOR PREVENTION OF SLUG FORMATION IN THE CASTING 0F FILM SUPPORT AND SHEETING Filed Dec. 27, 1960 2 Sheets-Sheet 1 42 m-lma-lmm- +A *L v Solvent ALFRED E. CZERKAS INVENTOR.

BY zzd/w ATTORNEYS Dec. 3, 1963 A. E. CZERKAS 3,112,523

METHOD AND APPARATUS FOR PREVENTION OF SLUG FORMATION IN THE CASTING OF FILM SUPPORT AND SHEETING Filed Dec. 27, 1960 2 Sheets-Sheet 2 ALFRED E CZERKAS BY rm/wars United States Patent Office 3,1 l2,5 2'8 Fatented Dec. 3, 1963 Filed Dec. 27, 1960, Ser. No. 78,560 7 Claims. (CI. 18-15) The present invention relates to an improved method and apparatus for manufacturing film base or sheeting, and particularly to an improved process for the prevention of, or removal of, slugs at the discharge slot of the hopper and to an improved apparatus for accomplishing this result.

As is well known in the art, cellulosic film or sheeting is formed by spreading a thin coating of a cellulose ester in suitable solvents onto an appropriate casting surface and permitting, or causing, the solvents to evaporate therefrom. The method generally employed consists in feeding the solution or dope from an appropriate hopper device onto a polished metallic surface of a slowly rotating wheel or drum. The solvent evaporates from the film more or less progressively as the wheel turns until, in less than one complete revolution, sufiicient solvent has been removed to permit the film being removed therefrom and conveyed to a windup or carried over other rolls or drums for further treatment.

The device usually employed for feeding the solution or dope to the casting wheel comprises a V-shaped hopper provided with a pair of blades mounted at an angle and spaced apart at the apex of the V to form a coating or discharge slot. Usually one or both of these blades is adjustable with respect to the other so as to provide means for controlling the thickness of the dope stream and, therefore, the sheet being formed.

Serious difiiculties often arise from the tendency of the cellulose ester solutions to form slugs or encrustations on the tips of either or both of the hopper blades, since, when these slugs form, they project down into the dope stream and give rise to uneven coating and the formation of streaks in the final product which render it unfit for use. This slugging is known to be due to the fact that small amounts of cellulose ester solution creep along the outer surfaces of the blade tips as it flows out of the hopper instead of being carried along in the dope stream onto the casting surface. The solvent evaporates from these small portions of dope and causes them to solidify, thus forming slugs. These slugs tend to increase in size due to further accretions of dope and eventually become sumciently large to project below the edges of the blades and cause the troublesome results mentioned. Also, any mechanical defects in the blade tips, or residues of dope left from previous cleaning processes, act as nuclei for slug formation and aggravate this problem.

The prior art has recognized that one Way of preventing the formation of slugs on the lips of coating hoppers is to enclose the area therearound to outside air and then maintain in such enclosure a high concentration of solvent air mixture. Also in some cases a liquid solvent has been fed to the ends of the hopper blades in an attempt to control slug formation at these particularly critical points. While these methods have been successful to some extent they have fallen short of complete blade tip slug control for one or more of the following reasons. First, efiicient bafiiing of the blade tip vicinity to form an enclosure capable of retaining a high solvent vapor concentration is difficult because of various casting wheel clearances required and because of the high degree of air turbulence that exists in this area. Secondly, in cases where vacuum is used behind the dope stream to keep it from vibrating and to obtain a quick pulldown of the stream onto the casting wheel, low concentrations of solvent vapor are drawn into the blade tip vicinity and slug conditions are augmented. Those prior art systems using enclosed areas around the blade tips in which an atmosphere of highly concentrated solvent vapor is maintained also require the use of an excessive quantity of solvent which must be recovered at considerable expense. This condition is particularly prevalent where small diameter casting wheels are used because in this case the depth of the vapor enclosure falls away from the discharge slot and, since the solvent vapor is about three times as heavy as air, in order to maintain a suitable vapor concentration in the enclosure around the discharge slot proper an excessive amount of solvent vapor is required to be continuously fed into the enclosure. Furthermore, while these prior art systems might have been considered to work satisfactorily under high flow coating conditions, or when using relatively low concentrations of film dope, when the volume of flow of the dope was decreased, as it would be when making very thin coatings, or when the concentration of the dope was increased, as it would be when less solvent was used therein, these prior art slug control systems failed and/ or required an excessive amount of solvent vapor to be fed into the enclosed areas embracing the blade tips in order to prevent the formation of slugs on the blades.

The primary object of the present invention is to provide a method and apparatus for casting film or sheeting by the use of which the formation of slugs on the blade tips of the hopper can be prevented or any slugs which might form can be readily removed, and at the same time no condensation problem is presented.

Another object is to provide a film casting apparatus wherein a controlled concentration of solvent vapor-air mixture is directed in a thin stream so that it blankets the blade tips at all times against stray machine air currents which would tend to evaporate the solvent from the dope streams at it emerges from the hopper discharge slot.

A further object is to provide a film casting apparatus which uses but a small fraction of the solvent required by prior art systems designed to accomplish the same result and one that requires no elaborate baffling system as required by the prior art.

Another object of the present invention is to provide a curtain of solvent vapor-air mixture so directed against the blade tips that it allows washing the blade tips free of any slugs that may be formed during a start up, or during a subsequent running of the machine, by merely raising the concentration of solvent in the vapor supply. This is possible with the present invention, whereas it would not have been possible with the prior art devices of which applicant is aware, because the vapor is so directed that any desired condensation of solvent therein can be controlled to occur on the blade tips where it is wanted rather than on the casting surface or parts of an enclosure area from which it could then drop onto the casting surface and form undesirable pits in the cast film or sheeting.

Still another object of the present invention is to provide a novel vapor supply for the blade tips of a dope hopper which will permit the casting operation to be momentarily shut down and then restarted without requiring a complete cleaning of the apparatus.

The novel features that -I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIG. 1 is an elevational view, particularly in section, showing an improved type of dope hopper constructed in accordance with a preferred embodiment of my invention in operative position on a film or sheet forming machine;

:FIG. 2 is an enlarged sectional elevation showing the vapor-air distributors at the bottom of the hopper, and taken substantially on line 2-2 of FIG. 3;

BIG. 3 is a bottom elevational view of the hopper with the vapor bars attached;

vFIG. 4 is a sectional detail taken on line 4-4 of FIG. 3 and showing how the end plates of the hopper are recessed to receive the ends of the vapor bars; and

FIG. 5 is a sectional detail of the lower end of a hopper showing another embodiment of vapor-air distributor which might be used in accordance with the present invention.

According to the present invention, the blade tips and the stream of dope emerging therefrom are blanketed at all times with a thin curtain of a solvent-air mixture in the vapor stage consisting approximately of 5070% solvent by weight. These curtains of solvent vapor prevent the penetration of stray air currents to the blade tips and thus eliminate evaporation of the solvent from the dope at these critical points which might tend to cause the formation of slugs, and they do this without need for elaborate baffling. Since these curtains of solvent vapor are directed right to the blade tips, any slug which might tend to form on the blade tips for any reason can be readily washed away by merely increasing the concentration of the solvent in the vapor to about 90% until the slug disappears and then dropping the concentration back to the normal operating range. Another advantageous feature of this vapor distributor system is that just before start up of the coating apparatus after a momentary shut down, the blade tips can be washed free of any dope encrustations or other foreign matter by merely increasing the concentration of solvent in the vapor. This also permits the apparatus to be momentarily shut down for any reason and be again started up without having to tear down and clean the whole hopper, a tedious and lengthy procedure which is now necessary with conventional casting apparatus. Since with the present invention the solvent vapor is directed right in contact with the blade tips, any condensation of solvent which is formed when the concentration of the dope is increased to or above 90% will be formed on the blade tips where it can act to dissolve any slug which might have formed on the tip, or dissolve any slug of dope which might have been purposely formed as a stopper in the discharge slot upon the complete shut down of the apparatus. This condensed solvent will dissolve away any slug formed on the hopper blade tips, or in the discharge slot, and will be carried away with the dope stream so that it will not fall onto the casting surface and form pits in the sheet being cast. This system of solvent vapor distribution has been found to give as good or better results in slug prevention than the prior art devices which attempt to form an enclosure around the discharge slot of the hopper and casting wheel by complicated bathing arrangements and then maintain a solvent vapor in this enclosed area, and does this with the use of but a small fraction of the solvent required by the prior art systems. This new solvent vapor distributor system not only requires the use of less sol-vent, but it does away with the need for expensive arrangements of baffies. The use of less solvent also reduces the danger of obtaining excessive explosive mixtures which has always been a great concern in the film casting art.

Referring to the drawings, the numeral designates a device for feeding the cellulosic solution or dope which, for example, may be a solution of cellulose nitrate or acetate in a suitable volatile solvent, onto the surface or wheel of a drum 11. This feeding device may conveniently be referred to as a dope hopper, the cellulose solution as a dope and the wheel or drum as the casting wheel, since these are terms customarily used in the art. The hopper may be adjustably mounted or suspended over the wheel by means, not shown, in such a manner that it can be lowered to or raised from the casting surface as occasion may require.

The hopper 10, as shown, comprises a V-shaped trough provided at the lower part of its front wall 1'2 with a blade 13 set into a recess in the hopper wall. Cooperating with the blade 13 is a blade 14 fitting into a recess in the rear wall 15 and provided with a plurality of adjusting screws 16 by which it may be moved toward and away from the blade 13, thereby regulating the width of the discharge slot 28 between the blades :13 and 14- and hence the thickness of the stream of dope which flows from the hopper onto the casting surface. 'It will be noted that these blades project downwardly into the space below the hopper and that even in the case of the back blade 14 there is always a small space between the edge of the blade and the wheel surface. The ends of the hopper are closed by end plates 29. The stream of dope emerging from the discharge slot of the hopper is deposited on the surface of wheel '11, which is continuously rotated in the direction of the arrow and forms a sheet 17 thereon. This sheet of freshly deposited dope sets up on the wheel as it rotates due to the loss of solvents, the gelled or set film then being removed or stripped from the wheel at point 18 and led over guide roll 19 to a wind-up station or through further curing stations, not shown. A casing 20 may surround the major portion of the wheel periphery to maintain warm air or other evaporative medium in contact with the sheet of film to accelerate the evaporation of solvents therefrom. Suitably conditioned air may be introduced into this casing at inlet 21 and be exhausted therefrom through outlet 22, the air flowing counter to the direction of movement of the wheel, to a solvent recovery system not shown. The present invent-ion is not limited in any way to the type of dope which is cast to form the film support or sheeting, but it is useful with any dope which contains a volatile solvent which, when it evaporates prematurely, tends to cause undesirable slugs to form on the tips of the hopper blades. The following are examples of different dopes which might be cast by the present apparatus.

Example 1 Cellulose acetate (43.5 acetyl) parts 100 Methylene chloride parts 426 Cyclohexane do 42 520 Methyl alcohol do.. 52 Triphenyl phosphate parts 15 Viscosity at F. poises 600 Example 2 Cellulose acetate (43.5% acetyl) parts Methylene chloride parts 490 55 Methyl alcohol do 60 0 Triphenyl phosphate parts Viscosity at 80 F. poises 450 Example 3 Cellulose acetate butyrate (17% butyryl) "parts" 100 Methylene chloride parts 280 Cyclohexane do 35 350 Butyl alcohol -4 do 35 Triphenyl phosphate "parts... 8 Viscosity at 80 F. poises 520 Organic solvent vapors evaporating from the cast dope.

on one or both sides of the discharge slot for directing a thin curtain of solvent vapor-air mixture down the blade tips to curtain the dope stream on one or both sides as it emerges from the hopper. Since the solvent vaporair distributors on both sides of the hopper are identical, except possibly for one dimension, the corresponding parts thereof will be referred to by the same reference numerals and only one will be specifically described. As best shown in FIGS. 2 and 3, these solvent vapor-air distributors may comprise a vapor bar or pan 3%, the back wall 31 of which has a flange 32 by means of which the bar is attached to the underside of rear and front walls -15 and 12, respectively, of the hopper by screws 28'. The bottom wall 33 of the vapor bar or pan is spaced from the bottom walls of the hopper and blade tips and has a front edge 33 which terminates short of planar surfaces 34 and 34' on the outside of each of the rear and front blade tips, respectively, which converge toward the discharge slot of the hopper. These front edges of the bar or pan are tapered parallel to these planar surfaces of the blade tips to form slit type orifices 35 and 35', which extend the full length of the hopper blades and are directed toward the discharge slot. The ends of the vapor bars or pans may be enclosed by end Walls 27, or as described below the ends or" the bars may be left open and abut the end walls 29 of the hopper to complete the enclosure of the chamber formed by the bars and the bottom of the hopper. Within each vaporair distributor is mounted a tubing 36, which extends substantially the full length of the distributor. This tubing is provided with a plurality of longitudinally spaced holes 37 through which a solvent vapor-air mixture fed into the tubing will be introduced into the distributor so that a uniform flow of the vapor-air mixture will pass through the orifices 35 and 35.

So that a solvent vapor-air mixture of adjustable concentration can be fed into the distributors under pressure, I provide a mixing device as shown at 4% in FIG. 1. The desired mixture of solvent vapor and air is regulated by valves 41 and 42, and is fed into the distributors through inlet pipe 43 which may enter the end of the tubing 36 as shown, or it may enter the tubing at any point therealong as desired. While for purposes of simplicity I have shown only one mixing device connected to one distributor, it is pointed out that both distributors may be supplied by the same mixing device, or if it is found desirable to control the concentration of the solvent vapor-air mixture to each blade tip individually for any reason, then each distributor can be supplied by its own vapor-air mixing device.

Under operating conditions a solvent vapor-air mixture in the vapor stage, and consisting of approximately 50-70% solvent by weight, is supplied to the distributors. The vapor-air mixture is forced out of the distributors by virtue of its pressure through the orifices 35 and 35 along the planar blade surfaces and directly to the blade tips as shown by the directional arrows. This thin stream of vapor-air mixture curtains the dope stream on all sides as it emerges from the hopper. This reduces the solvent concentration gradient between the surrounding air and the emerging .dope stream at the blade tips and consequently reduces the solvent loss and slug formation at this critical point. The uniqueness in this apparatus lies in the fact that the solvent vapor-air mixture is fed so that it blankets the blade tips and the dope stream emerging therefrom at all times. Stray machine air currents due to poor bafiling do not affect the concentration at the blade tips because the stable curtain of vapor-air mixture at this point is not penetrated by these stray air currents. Auxiliary vapor-air supplies which are part of prior art machines of this type feed vapor-air mixtures to enclosures embracing the blade tips but these mixtures are affected by stray air currents and require expensive baflie arrangements which the present invention does not require. Furthermore, these prior art systems do not function satisfactorily with casting apparatus using small diameter casting wheels because of the fact that the solvent vapor-air mixture is heavier than air and tends to fall away from the discharge slot of the hopper and thus require an excessive introduction of solvent vapor-air mixture into the enclosure in order to maintain the necessary concentration at the discharge slot where it is needed. With applicants invention, on the other hand, this is no problem because the solvent vaporair mixture completely blankets the discharge slot of the hopper without requiring an enclosed area at this point and the fact that the solvent vapor is much heavier than air is made use of in the present invention since its tendency to flow downwardly aids in forming the desired curtain. It will be readily appreciated that this type of vapor-air distribution is readily applicable to casting apparatus using small diameter casting surfaces and to bring out this point more clearly applicant has shown in FIG. 2 the casting surface having a small radius, in fact one rather exaggerated in this respect as compared to the showing in HG. 1. In contrast to the prior art devices which require excessive quantities of solvent to prevent slug formation according to their teachings, the vapor-air distributors attached to the blades according to the present invention require a small fraction of the solvent required by presently used slug prevention systems. For example, using the present invention in place of the solvent vapor-air distributor system shown in U.S. Patent 2,831,211 at the second stage of a multiple casting machine showed a saving of 135,000 pounds of solvent per year per machine and eliminated the elaborate baffling arrangement required on the patented machine.

In has been found that the present invention gives the desired results if the orifices 35 and 35' are .020.070 inch in width and are spaced anywhere between A to inch form the blade tips. Because of the constant flow of a current of vapor-air mixture it has been found that a higher concentration of solvent can be used, than is possible in prior art systems without encountering any undesirable condensation problems for the reason that there is no large volume of solvent vapor-air mixture which might come in contact with cold spots of a chamber enclosing such a large volume as in the prior art instructions. The present invention is particularly advantageous in cases where the volume of flow of dope is slow (as would be the case where very thin films are being formed) and/orthe concentration of dope is high (low solvent content) these conditions being very critical to the formation of slugs and not readily accommodated for by prior art systems without the use of excessive amounts of solvent vapor-air mixtures which introduce undesirable condensation problems.

In the preferred embodiment of the invention, I have found it desirable to have the discharge orifices 35 and 35' of the vapor-air distributors longer than the discharge slot of the hopper and so arranged that they will overhang the discharge slot at both ends as shown in FIG. 3. This is accomplished by recessing the end plates 29 of the hopper, as shown at 5% in FIGS. 3 and 4, and having the ends of the vapor bars or pans 30' extend into these recesses. If the ends of the vapor bars or pans are made to abut the end walls of the hopper, it will be apparent that there need be no end Walls on the pans per se since the end walls of the hopper will close the ends of the bars or pans. This added length of the distributor orifices 35 and 35' insures that the ends of the emerging dope stream are blanketed with the solvent vapor-air mixture, and is particularly desirable in cases where a vacuum is used at the back side of the emerging dope stream to steady it during the time it is suspended between the discharge slot and the casting surface. However, without going beyond the scope of the present invention, the orifices 35 an 35 of the distributors could be made equal, or substantially equal, in length to the discharge slot of the hopper in which case the ends of the vapor bars or pans could abut the end plates of the hopper without requiring recesses therein, or the bars or pans could have end walls of their own as shown.

Rather than having the solvent vapor-air distributors formed integrally with the hopper as shown in the preferred embodiment, the present invention could be carried out by the use of vapor-air distributors which are separate from the hopper as shown in FIG. 5. Here I have shown the vapor-air distributors as a separate elongated chamber 69 having an orifice 61 arranged to direct a thin curtain of vapor-air mixture against the outer planar surface of a blade tip at such angle that the stream of vapor-air mixture will flow down the blade surface to blanket the emerging dope stream. In this instance the chamber 6i! and its orifice 6 1 would be substantially equal to the length of the discharge slot and would be fed with a solvent vapor-air mixture whose concentration could be readily controlled.

Under operating'conditions, I have found that if a solvent-air mixture in the vapor stage consisting of approximately 50-70% solvent by weight is supplied to the distributors at between .5 c.f.m. to 1.5 c.f.n1., slugging problems will be nonexistent. Under normal conditions the vapor-air mixture concentration is adjusted so that it has a dew point slightly above the temperature of the blade tip, but for washing away a slug which appears to be forming or at start up after slug formation, the solvent vapor concentration of the mixture is increased to a point where it has a dew point below the temperature of the blade tip to obtain a condensation of solvent on the blade tips to dissolve the slug. A 90% concentration of solvent in the mixture will generally accomplish this result. It will be seen that with the present system of solvent vapor-air mixture distribution a desired condensation of solvent can be readily obtained at the critical slug forming points (blade tips) without the danger of undesirable condensation of solvent at other points surrounding the discharge end of the hopper.

While it is generally desirable to use a vapor-air distributor in conjunction with each hopper blade as shown and described, it is conceivable that under certain casting conditions a slugging problem might be troublesome at only one blade so that only one distributor rather than two would be required. Accordingly, my invention is not to be limited to the use of a vapor-air distributor on each hopper blade tip, but is intended to cover the idea of using but one distributor in combination with only one blade tip.

While I have shown and described certain specific embodiments of my invention, I am aware that many modiifications thereof are possible. My invention, therefore, is not to be limited to the precise details shown and described but is intended to cover all modifications coming within the scope of the appended claims.

Having thus disclosed my invention, what I claim is new and novel and desire to secure by Letters Patent of the United States is:

1. In a film or sheet forming apparatus the combination with a casting surface: means for continuously moving the same; a dope hopper including a pair of blades having tips which are spaced apart to form an elongated discharge slot spaced from said casting surface through which a thin stream of flowable dope composed of solids dissolved in a volatile solvent is adapted to be continuously deposited onto said casting surface, the outer face of at least one of said blade tips being substantially planar and inclined toward said discharge slot; of means for continuously preparing a mixture of vapor of said solvent and air whose concentration is such that the mixture has a dew point above the operating temperature of said blades; means for continuously =feeding said vapor and air mixture as a thin stream having a width at least equal to the length of said discharge slot and directing said stream against and crosswise of said outer planar surface in a direction toward said discharge slot to form a thin curtain of said vapor and air mixture which flows along said planar surface and along the side of the dope stream passing over said blade tip and prevents the loss of solvent from said dope streamand the formation of dope slugs on said blade tip.

2. In a film or sheet forming apparatus the combination with a casting surface; means for continuously moving the same; a dope hopper including a pair of blades having tips which are spaced apart to form an elongated discharge slot spaced from said casting surface through which a thin stream of flowable dope composed of solids dissolved in a volatile solvent is adapted to be continuously deposited onto said casting surface, the outer face of both blade tips being substantially planar and converging toward said discharge slot; of means for continuously preparing a mixture of vapor of said solvent and air whose concentration is such that the mixture has a dew point above the operating temperature of said blades; means for continuously feeding said vapor and air mixture into two thin streams each having a width at least equal to the length of said discharge slot and directing one of said streams against and crosswise of each of said outer planar surfaces in a direction toward said discharge slot to form two thin curtains of said vapor and air mixture which flow along said planar surfaces and along the sides of the dope stream passing over said blade tips and prevents the loss of solvent from said dope stream and the formation of dope slugs on said blade tips.

3. A film or sheet forming apparatus according to claim 1 in which said last mentioned means comprises a closed vapor-air distributor disposed between the bottom of said hopper and the casting surface on that side of the hopper having the blade tip with said planar outer surface, said distributor having a narrow orifice slot extending the full length of said blade tip for directing a thin stream of solvent vapor-air mixture against the planar surface of said blade tip at a point above said dis a charge slot and toward the same; and means for introducing said solvent vapor and air mixture into said distributor under pressure so that a substantially uniform stream will issue from the orifice throughout its entire length.

4. A film or sheet forming apparatus according to claim 1 in which said last mentioned means comprises an offset vapor bar fixed to the underside of said hopper and cooperating therewith to provide a closed vapor and air mixture distributor extending longitudinally the full length of said blade tip, one edge of said vapor bar terminating short of the planar surface of the blade tip at a point above said discharge slot to form an orifice through which a thin stream of vapor and air mixture is directed down said blade tip toward the discharge slot, and means for introducing said solvent vapor-air mixture into said distributor under pressure so that a substantially uniform stream of vapor and air mixture will issue from said orifice.

5. In the method of casting film or sheeting comprising continuously flowing a thin stream of flowable dope composed of solids dissolved in a volatile solvent through a discharge slot of a hopper having an operating temperature onto a casting surface spaced therefrom and moving continuously relative thereto; the steps of preparing a mixture of vapor of said solvent and air whose concentration is such that the mixture has a dew point above the temperature of said hopper, and flowing a thin stream of said solvent vapor and air mixture across one side of the full length of the discharge slot and the side of the dope stream passing thereover to blanket said edge of the discharge slot and said side of the dope stream as it emerges therefrom from contact with an atmosphere which would tend to allow evaporation of thesolvent from I v the dope stream and produce slugs of dope on the edge of the discharge slot.

6. The method of casting film or sheeting according to claim 5 in which a thin stream of said volatile solvent vapor and air mixture of controlled concentration is flowed down both sides of the full length of the discharge slot and both sides of the dope stream issuing therefrom $3 to prevent the production of slugs on either edge of the discharge slot.

7. The method of casting film or sheeting according to claim 5 including the step of selectively adjusting the concentration of said vapor and air mixture until it has a dew point below the temperature of said hopper so that the solvent Will condense out on the edge of said discharge slot and dissolve and wash away any slug of dope which might have formed on the edge of the discharge slot.

References (fitted in the file of this patent UNITED STATES PATENTS Davidson Aug. 18, 1936 Fordyce Sept. 8, 1942 Fordyce et al Sept. 8, 1942 Nadeau Feb. 13, 1945 Czerkas May 8, 1962 

5. IN THE METHOD OF CASTING FILM OR SHEETING COMPRISING CONTINUOUSLY FLOWING A THIN STREAM OF FLOWABLE DOPE COMPOSED OF SOLIDS DISSOLVED IN A VOLATILE SOLVENT THROUGH A DISCHARGE SLOT OF A HOPPER HAVING AN OPERATING TEMPERATURE ONTO A CASTING SURFACE SPACED THEREFROM AND MOVING CONTINUOUSLY RELATIVE THERETO; THE STEPS OF PREPARING A MIXTURE OF VAPOR OF SAID SOLVENT AND AIR WHOSE CONCENTRATION IS SUCH THAT THE MIXTURE HAS A DEW POINT ABOVE THE TEMPERATURE OF SAID HOPPER, AND FLOWING A THIN STREAM OF SAID SOLVENT VAPOR AND AIR MIXTURE ACROSS ONE SIDE OF THE FULL LENGTH OF THE DISCHARGE SLOT AND THE SIDE OF THE DOPE STREAM PASSING THEREOVER TO BLANKET SID EDGE OF THE DISCHARGE SLOT AND SAID SIDE OF THE DOPE STREAM AS IT EMERGES THEREFROM FROM CONTACT WITH AN ATOMSPERE WHICH WOULD TEND TO ALLOW EVAPORATION OF THE SOLVENT FROM THE DOPE STREAM AND PRODUCE SLUGS OF DOPE ON THE EDGE OF THE DISCHARGE SLOT. 